netblock

A netblock is a contiguous range of IP addresses allocated to a single organization or network operator for the purposes of addressing and routing. In IPv4, netblocks are commonly expressed in CIDR notation, such as 203.0.113.0/24, which designates 256 addresses; in IPv6, prefixes are expressed similarly, for example 2001:db8::/32.

Allocation and administration: Netblocks are allocated by regional Internet registries (RIRs) to Internet service providers, enterprises, and sometimes end-user organizations. The RIRs coordinate distribution, maintain registration records, and promote aggregation to reduce the size of global routing tables. Block sizes vary by policy and region, and smaller blocks are often subdivided and assigned to customers through subnets.

Routing and use: Netblocks are announced in the Border Gateway Protocol (BGP); routers use the prefix to forward traffic toward the owning network. Netblocks enable routing policy, traffic engineering, and access control, and they support network troubleshooting and monitoring. They are also used in security contexts to block or allow traffic from specified ranges.

Management and transfers: Netblocks can be reassigned or transferred between organizations, with ownership reflected in registration databases. IPv4 netblocks are a legacy resource with limited supply, while IPv6 netblocks offer substantially larger address space and more flexible allocation. Accurate documentation and up-to-date registration information are important for network governance and interoperability.